| Market forces as well as better tool support and upgradability of commercial off-the-shelf (COTS) components have made it necessary to consider using them for building real-time applications. In this dissertation, we investigate a practical approach for developing distributed real-time applications in an environment consisting of COTS components with minimal support for real-time. We concentrate on the design of a software architecture at the middleware level, and resource management and scheduling techniques to reduce the unpredictability inherent in these type of components. Instead of modifying the COTS components, we provide solutions that sit on top of the COTS components.; In the first part of the dissertation, we discuss the limitations and real-time features of modern general purpose operating systems. In particular, a characterization of Windows NT is presented from the perspective of real-time constraints. We systematically arrive at guidelines and recommendations that will be useful for real time system designers as they build applications using NT. We find that the unpredictable part of NT can be minimized by the use of a user-level scheduling scheme.; In the second part of the dissertation, we present solutions for providing end-to-end predictability in distributed systems. The issues that need to be addressed include: (1) mapping application real-time requirements into requirements imposed on the system schedulable entities, (2) ensuring predictable execution of tasks in the face of priority inversions, limited OS level real-time scheduling support, and limited number of priorities, and (3) integrating real-time and non-real time tasks in the same computing environment. Our experimentally validated solution is based on a user-level scheduling scheme for communicating tasks. The scheduling scheme consists of a combination of server-based execution, rate control of message communication and a modified form of dual priority scheduling.; For concreteness, the above scheduling scheme is designed and implemented in the context of MidART, a Middleware and network Architecture for distributed Real-Time systems. MidART provides a distributed real-time application a development package with an easy-to-use programming interface for real-time data acquisition and communication. To enhance the portability and interoperability of MidART, We experimentally evaluate this service to demonstrate the feasibility of our approach. |
